This cap is made to fit the outside of 4 inch PVC schedule 40 pipe it is not intended for thin wall storm drains but 4 1/2 inches sounds about right. Plumbing diameters are measured from the inside dimension.
Should, as long as it fits the dimensional range of the cap.
Yes it was
It probably will. If you tighten it down fully and it still feels loose you can wrap the duct with duct tape a few times to increase it's circumference.